1. Field of the Invention
The present invention relates to an excitation controller and an excitation control method for stabilizing voltage in an electric power system.
2. Description of the Prior Art
Prior art excitation controllers detect a high-side voltage of a transformer connected to a synchronous machine or generator and then control the synchronous machine according to the detected voltage such that the high-side voltage has a constant value, in order to improve the stability of the electric power system, as disclosed in Japanese patent application publication (TOKKAIHEI) No.4-79798, for example.
Referring next to FIG. 23, there is illustrated a block diagram showing the structure of a prior art excitation controller as disclosed in Japanese patent application publication (TOKKAIHEI) No.4-79798. As shown in the figure, a first potential transformer or PT 102 detects an output terminal voltage V.sub.G of a synchronous machine 101, and a subtracter 104 subtracts the detected output terminal voltage V.sub.G from a reference voltage r.sub.G set by a voltage setter 103. A reduced gain circuit 105 then multiplies the result of the subtraction from the subtracter 104 by a gain .beta..
A second potential transformer or PT 109 detects a transmission voltage V.sub.H on a transmission bus 106 (i.e., a high-side voltage of a transformer 108 connected, by way of a breaker 107, to the transmission bus 106), and a second subtracter 111 subtracts the detected transmission voltage V.sub.H from a transmission reference voltage r.sub.H set by a high-side voltage setter 110. A high-voltage gain circuit 112 then multiplies the result of the subtraction from the second subtracter 111 by a gain K.sub.H.
Then an adder 113 adds the result of the multiplication from the reduced gain circuit 105 and the result of the multiplication from the high-voltage gain circuit 112 together. An automatic voltage regulator or AVR 114 generates a timing signal for controlling the rectifying timing of an exciter 115 based on the result of the addition from the adder 113. The exciter 115 receives the timing signal and then supplies a field current to a field winding 116 of the synchronous machine 101 according to the timing signal.
In this way, the prior art excitation controller detects the high-side voltage of the transformer 108, and then controls the synchronous machine such that the transmission voltage on the transmission bus 106 remains constant.
A problem with a prior art excitation controller constructed as above is that although it can keep the high-side voltage of a transformer connected to a synchronous machine constant, in addition to an excitation control panel on which an AVR and an adder are mounted, another control panel is needed for mounting a high-side voltage setter, a potential transformer, a reference value correction circuit, and so on thereon, thus increasing the manufacturing cost of the electric power system.
Another problem is that since a transmission bus is located at a considerable distance from an electric power plant building for housing the synchronous machine, the transformer, and so on, a long cable susceptible to noise is needed to connect an excitation control panel located in the electric power plant building to a PT located in the vicinity of the transmission bus, thus decreasing the reliability of the electric power system.
A further problem is that an expensive PT is needed to detect the transmission voltage on the transmission bus and hence the manufacturing cost of the electric power system is increased.
A still further problem is that since when a transformer equipped with a tap changer for changing taps under load conditions is provided, the tap-changer-equipped transformer including an automatic tap control function, changing taps changes the transformation ratio, it is difficult to maintain the output terminal voltage of the generator at a nominal value and to keep the transmission voltage on the transmission bus constant. Another problem is that since the reactance X.sub.t of the tap-changer-equipped transformer changes with a change in taps of the tap-changer-equipped transformer as this results in a change in the ratio of a cross-current prevention reactance X.sub.DR for preventing any cross current from flowing between the synchronous machine and another synchronous machine, to the reactance X.sub.t of the tap-changer-equipped transformer, a cross current can flow among a plurality of synchronous machines including the above-mentioned synchronous machines, which operate in parallel with one another, if the plurality of synchronous machines have different transformation ratios.